1. Field of the Invention
This invention relates to a standing wave type linear accelerator, and particularly to its high frequency power automatic frequency control.
2. Description of the Prior Art
FIG. 1 is a block diagram of a conventional high frequency power automatic frequency controller of a standing wave type linear accelerator shown in, for example, Japanese Patent Laid-open No. 53-117198. In FIG. 1, reference numeral 1 is a high frequency generator which generates high frequency power, 2 a waveguide connected to the output side of the high frequency generator 1 for conducting the high frequency power generated, 3 a circulator connected to the output side of the waveguide 2 for conducting the high frequency power, 4 an electron gun for generating electrons, 5 a standing wave type linear accelerating tube interconnected to the circulator 3 for accelerating electrons from the electron gun 4, 6 is a water load connected to the output side of the circulator 3 for absorbing the high frequency power reflected from the accelerating tube 5 via the circulator 3, 7a an attenuator inserted into the output side of the waveguide 2 if necessary, 7b an attenuator inserted into the output side of a coupling portion between the circulator 3 and the water load 6, 8 a delay line connected via the attenuator 7a to the waveguide 2 for delaying the incident high frequency power, 9 a variable phase shifter for changing a phase of the incident high frequency power in the output side of the delay line 8, 10 a hybrid ring, that is, a high frequency mixer connected to the variable phase shifter 9 and at the same time, connected via the attenuator 7b to the circulator 3, 11a and 11b are high frequency diodes connected to respective output terminals of the hybrid ring 10, 12a and 12b are attenuators connected to the high frequency diodes 11a and 11b if necessary, respectively, 13 a differential amplifier whose input terminals are connected to the attenuators 12a and 12b, respectively, and 14 a servomotor connected across output terminals of the differential amplifier 13 for mechanically adjusting an oscillation frequency of the high frequency generator 1.
A conventional high frequency power automatic frequency controller is constituted as described above, and hereinafter the operation thereof will be described. The high frequency power V.sub.o generated by the high frequency generator 1 is supplied to the accelerating tube 5 via the waveguide 2 and the circulator 3 for incidence. The high frequency power reflected by the accelerating tube 5 is conducted to the water load 6 via the circulator 3 and absorbed therein. One part of the high frequency power V.sub.o is extracted by the waveguide 2 and conducted via the delay line 8 and the variable phase shifter 9 to the hybrid ring 10 as an incident high frequency power V.sub.I. One part of the reflected high frequency power from the accelerating tube 5 is taken out from the coupling portion between the circulator 3 and the water load 6 and sent to the hybrid ring 10 as a reflected high frequency power V.sub.R. The incident high frequency power V.sub.I and the reflected high frequency power V.sub.R are mixed in the form of a vector by the hybrid ring 10. The outputs V.sub.1 and V.sub.2 are respectively detected by the high frequency diodes 11a and 11b and input via the attenuators 12a and 12b to the differential amplifier 13. An output .vertline.V.sub.1 .vertline.-.vertline.V.sub.2 .vertline. of the differential amplifier 13 corresponds to a shift in frequency .DELTA.fo between the incident and reflected high frequency power V.sub. I and V.sub.R, that is, a phase shift .DELTA..phi..sub.o and adjusts a tuner (not shown) of the high frequency generator 1 by giving normal rotation or reverse rotation to the servomotor 14 in accordance with the polarity of the phase shift, allowing the oscillation frequency to be controlled. The correspondence of the polarity of the output .vertline.V.sub.1 .vertline.-.vertline.V.sub.2 .vertline. to the polarity of the phase shift .DELTA..phi..sub.o between the incident high frequency power and the reflected high frequency power will be described below. Hereinafter, necessary constants and so forth are omitted for the sake of convenience in order to describe the correspondence while paying attention to the phase shift .DELTA..phi..sub.o. In the constitution shown in FIG. 3, when the frequency of the high frequency power V.sub.o is equal to an optimal acceleration frequency fo of the accelerating tube 5, the reflected high frequency power V.sub.R is behind the incident high frequency power V.sub.I by .pi./2 radians. Accordingly, the delay in phase of the reflected high frequency power V.sub.R at the time when the frequency of the high frequency power V.sub.o is fo+.DELTA.fo can be represented by .pi./2+.DELTA..phi..sub.o. Where, in this case, .DELTA..phi..sub.o is in a range of -.pi./2&lt;.DELTA..phi..sub.o &lt;.pi./2. Accordingly, when the phase of the high frequency power V.sub.o is made to delay by .pi./2 radians by adjusting the delay line 8 and the variable phase shifter 9, the relation between the high frequency power V.sub.o, and high frequency power inputs V.sub. I and V.sub.R of the hybrid ring 10 can be represented as follows. ##EQU1## where V.sub.o, V.sub.I, and V.sub.R are amplitudes, .omega. is an angular frequency of the high frequency power, and t is time).
Accordingly, from the characteristics of the hybrid ring 10 the V.sub.1 and V.sub.2 are represented as follows. ##EQU2## When these high frequency powers are detected by the high frequency diodes 11a and 11b, the outputs become the absolute values of the expressions (4) and (5). Accordingly, they are represented as follows. ##EQU3##
Since .vertline.V.sub.1 .vertline.&gt;0 and .vertline.V.sub.2 .vertline.&gt;0, and the polarity of .vertline.V.sub.1 .vertline.-.vertline.V.sub.2 .vertline. is same as that of .vertline.V.sub.1 .vertline..sup.2 -.vertline.V.sub.2 .vertline..sup.2, the following expression holds. EQU .vertline.V.sub.1 .vertline..sup.2 -.vertline.V.sub.2 .vertline..sup.2 =-2V.sub.I V.sub.R sin(.DELTA..phi..sub.o) (8)
Accordingly, it is found that the following expressions hold from the expression (8). ##EQU4##
As described above, the phase of the reflected high frequency power from the accelerating tube 5 is detected to apply negative feedback to the high frequency generator 1 and make the oscillation frequency of the high frequency power follow the variation of the optimal acceleration frequency of the accelerating tube 5 due to temperature changes. Incidentally, in general, a linear accelerator utilizing a high frequency adopts a pulse operation system, and its high frequency output and its output beam both have a pulse waveform. FIG. 2 is a diagram showing a high frequency input detected waveform of the differential amplifier 13 at the terminals a and b in FIG. 1. V.sub.P1 and V.sub.P2 each show a pulse height, and .tau..sub.1 and .tau..sub.2 each show a pulth width, T shows a pulse repetitive interval time, and V.sub.av1 and V.sub.av2 each show a voltage value obtained by integrating a pulse and averaging the integrated value. Incidentally, V.sub.av1 and V.sub.av2 are represented by the following expressions (9) and (10). ##EQU5## Usually, the frequency control using a pulse operation system drives the servomotor 14 in such a manner that the differential output V.sub.av1 -V.sub.av2 of values V.sub.av1 and V.sub.av2 obtained by averaging pulse output waveforms becomes zero.
Since a conventional high frequency power automatic frequency controller uses a signal obtained by averaging a high frequency output detected waveform and the level of the averaged signal is small, the amplification factor of the differential amplifier is made large to drive the servomotor. Accordingly, there are problems that the offset adjustment for the differential amplifier is necessary, and the control circuit is prone to operate faultily due to drifts and noises caused by temperature changes. Even in the case where the control circuit operates faultily and the frequency is far away from the optimal acceleration condition, there are problems that the conventional high frequency power automatic frequency controller has not the function of stopping the operation of the controller, energy of the output beam is largely lowered, and the controller is in danger of continuing to be operated in a state in which no output beam can be obtained.